Nylon has been manufactured and used commercially for a number of years. The first nylon fibers were of nylon 6,6, poly(hexamethylene adipamide), and nylon 6,6 fiber is still made and used commercially as the main nylon fiber. Large quantities of other nylon fibers, especially nylon 6 fiber prepared from caprolactam, are also made and used commercially. Nylon fiber is used in yarns for textile fabrics, and for other purposes. For textile fabrics, there are essentially two main yarn categories, namely continuous filament yarns and yarns made from staple fiber, i.e. cut fiber.
Nylon staple fiber has conventionally been made by melt-spinning nylon polymer into filaments, collecting very large numbers of these filaments into a tow, subjecting the tow to a drawing operation and then converting the tow to staple fiber, e.g., in a staple cutter. The tow usually contains many thousands of filaments and is generally of the order of several hundred thousand (or more) in total denier. The drawing operation involves conveying the tow between a set of feed rolls and a set of draw rolls (operating at a higher speed than the feed rolls) to increase the orientation of nylon polymer in the filaments. Drawing is often combined with an annealing operation to increase nylon crystallinity in the tow filaments before the tow is converted into staple fiber.
One of the advantages of nylon staple fibers is that they are readily blended, particularly with natural fibers, such as cotton (often referred to as short staple) and/or with other synthetic fibers, to achieve the advantages derivable from such blending. A particularly desirable form of nylon staple fiber has been used for many years for blending with cotton, particularly to improve the durability and economics of the fabrics made from yarns comprising blends of cotton with nylon. This is because such nylon staple fiber has a relatively high load-bearing tenacity, as disclosed in Hebeler, U.S. Pat. Nos. 3,044,250; 3,188,790; 3,321,448; and 3,459,845, the disclosures of which are hereby entirely incorporated by reference. As explained by Hebeler, the load-bearing capacity of nylon staple fiber is conveniently measured as the tenacity at 7% elongation (T7), and the T7 parameter has long been accepted as a standard measurement and is easily read on an Instron machine.
The Hebeler process for preparing nylon staple fiber involves the nylon spinning, tow forming, drawing and converting operations hereinbefore described. Improvements in the Hebeler process for preparing nylon staple fiber have subsequently been made by modifying the nature of the tow drawing operation and by adding specific types of annealing (or high temperature treatment) and subsequent cooling steps to the overall process. For example, Thompson in U.S. Pat. Nos. 5,093,195 and 5,011,645 discloses nylon staple fiber preparation wherein nylon 6,6 polymer, having for example a formic acid relative viscosity (RV) of 55, is spun into filaments which are then drawn, annealed, cooled and cut into staple fiber having a tenacity, T, at break of about 6.8-6.9, a denier per filament of about 2.44, and a load-bearing capacity, T7, of from about 2.4 to 3.2. Such nylon staple fibers are further disclosed in the Thompson patents as being blended with cotton and formed into yarns of improved yarn strength. (Both of these Thompson patents are incorporated herein by reference in their entirety.)
Nylon staple fibers prepared in accordance with the Thompson technology have been blended into NYCO yarns (generally at a 50:50 nylon/cotton ratio) with these yarns being used to prepare NYCO fabrics. Such NYCO fabrics, e.g., woven fabrics, find application in military combat uniforms and apparel. While such fabrics have generally proven satisfactory for military or other rugged apparel use, military authorities, for example, are continually looking for improved fabrics which may be lighter in weight, lower in cost and/or more comfortable but still highly durable or even of improved durability.
One route to such fabrics of improved durability and comfort and lighter weight could involve the preparation of NYCO yarns, and fabrics made therefrom, wherein the nylon staple fibers used in yarn preparation have improved load-bearing capacity in comparison with existing nylon staple fibers. Fabrics prepared from yarns using such improved load-bearing nylon staple fibers could advantageously be made to have equivalent or even improved durability in comparison with currently used fabrics. Nylon staple fibers of increased load-bearing capacity could provide such desirable durability performance by being incorporated into lighter weight and/or lower cost fabric which potentially uses less of the nylon staple fiber than is currently employed in such fabrics.